High Resolution Touch Sensor Apparatus and Method

ABSTRACT

A sensor array ( 10 ) comprising a plurality of touch sensitive pixels, each pixel ( 12 ) comprising a capacitive sensing electrode ( 14 ) and a reference capacitor ( 16 ) connected in series with the capacitive sensing electrode ( 14 ) to provide an indicator voltage that is indicative of the proximity of a conductive object to be sensed.

FIELD OF INVENTION

The present invention relates to apparatus and methods, and moreparticularly to touch sensitive surfaces, methods for control and signalacquisition from such surfaces, and methods of manufacturing suchsurfaces.

BACKGROUND

Secure, verifiable authentication, of user identity is an increasinglyimportant part of all technology. To give just a few examples, it playsa part in:

-   -   user equipment (UE) used for communication and consumer access        to media content,    -   computer devices and systems which store and provide access to        sensitive data,    -   devices and systems used for financial transactions, access        control for buildings, and    -   access control for vehicles.

Biometric measurement of the user is now prevalent in all of thesecontexts and others. Biometric measures such as iris scanning, andfacial recognition are dependent on lighting and field of view of acamera. It may also be possible to circumvent such security measures bypresenting a video or photo of the user to the camera.

Fingerprint sensors have been thought of as being more secure, but it ispossible also to overcome the security they provide, and themanufacturing requirements of such sensors makes it difficult tointegrate them into other electronic devices such as mobile telephonesand other UEs. In particular, fingerprint sensing demands very highresolution—at least hundreds of pixels per inch.

One example of such a sensor is Apple Inc's Touch ID®. This sensor isbased on a laser-cut sapphire crystal. It uses a detection ring aroundthe sensor to detect the presence of the user's finger. The Touch ID®sensor uses capacitive touch sensing to detect the fingerprint, and hasa 500 pixel per inch (PPI) resolution.

Capacitance sensors such as these use capacitive effects associated withthe surface contours of the fingerprint. The sensor array pixels eachinclude an electrode which acts as one plate of a capacitor, the dermallayer (which is electrically conductive) acts as the other plate, andthe non-conductive epidermal layer acts as a dielectric. The capacitanceis greater where the dermis is closer to the pixel electrode, and so thesurface contours of the skin can be sensed by measuring the capacitanceof each pixel (e.g. based on the charge accumulated on the pixelelectrode) and assembling an image from those pixels.

Both passive matrix and active matrix capacitive touch sensors have beenproposed. Most so-called passive capacitive touch sensing systems use anexternal driving circuit (such as an integrated circuit, IC) to drive amatrix of passive electrodes, and a separate readout circuit (e.g. anIC) to readout charge stored on these electrodes during the drive cycle.The stored charge varies dependent on the tiny capacitance changes dueto touch events. Passive electrode systems are sensitive toenvironmental noise and interference.

Active matrix capacitive touch sensors include a switching element ineach pixel. The switching element may control a conduction path betweenthe capacitive sensing electrode in the pixel, and an input channel toan analogue to digital converter (ADC) in a read-out circuit. Typicallyeach column of pixels in an active array is connected to one such inputchannel. The charge stored in the array can thus be read from the activematrix by controlling the switching elements to connect each row ofpixels, one-by-one, to the ADC.

Each pixel needs to be connected to the read-out circuit, and all of thepixels of each column are effectively connected in parallel. Theparasitic capacitance associated with each pixel therefore combinesadditively. This places an inherent limit on the number of pixels thatcan be combined together in any one column. This in turn limits the sizeand/or resolution of a capacitive touch sensor.

There thus remains a significant unmet commercial need for large areahigh resolution touch sensors.

SUMMARY

Aspects and examples of the invention are set out in the claims and aimto address at least a part of the above described technical problem, andother problems.

In an aspect there is provided a sensor array comprising a plurality oftouch sensitive pixels, each pixel comprising: a capacitive sensingelectrode for accumulating a charge in response to proximity of aconductive object to be sensed; a reference capacitor connected inseries with the capacitive sensing electrode so that, in response to acontrol voltage, an indicator voltage is provided at the connection (18)between the reference capacitor and the capacitive sensing electrode toindicate the proximity of the conductive object to be sensed. Thisarrangement may reduce or overcome the problem associated with parasiticcapacitance which may occur in prior art touch sensors.

Each pixel may comprise a sense VCI (voltage controlled impedance)having a control terminal connected so that the impedance of the senseVCI is controlled by the indicator voltage. Typically the sense VCIcomprises at least one TFT (thin film transistor) and the conductionpath of the VCI comprises the channel of the TFT. A conduction path ofthe sense VCI may be connected to a first plate of the referencecapacitor (16), and the control terminal of the first VCI is connectedto the second plate of the reference capacitor. At least one plate ofthe reference capacitor may be provided by a metallisation layer of athin film structure which provides the sense VCI.

The conduction path of the sense VCI may connect the first plate of thereference capacitor, and so also the control voltage, to an input of areadout circuit. This may enable the circuitry which provides thecontrol voltage also to provide the basis for the output signal of thepixel. This may further address problems associated with parasiticcapacitance and signal to noise ratio in prior art touch sensors. Analternative way to address this same problem is to arrange theconduction path of the sense VCI to connect a reference signal supply toan input of a readout circuit. The reference signal supply may comprisea constant voltage current source. Thus, modulating the impedance of thesense VCI of a pixel controls the current from that pixel to the inputof the read-out circuit.

A select VCI may also be included in each pixel. This may be connectedso that its conduction path is connected in series between theconduction path of the sense VCI and the reference signal supply. Thus,switching the select VCI into a non-conducting state can isolate thesense VCI from the reference signal input, whereas switching the selectVCI into a conducting state can enable current to flow through the pixel(depending on the impedance of the sense VCI). A control terminal of theselect VCI may be connected for receiving the control voltage, e.g. froma gate drive circuit.

Each pixel may comprise a reset circuit for setting the control terminal(22) of the sense VCI to a selected reset voltage. The reset circuit maycomprise a reset VCI. A conduction path of the reset VCI is connectedbetween a second plate of the reference capacitor and one of (a) a resetvoltage; and (b) a first plate of the reference capacitor. A controlterminal (32) of the reset VCI may be connected to another pixel of thesensor for receiving a reset signal (e.g. from a channel of a gate drivecircuit which is connected to the control terminal of the select VCI ofa pixel in another row of the array). The reset signal may be configuredto switch the reset VCI into a conducting state, thereby to connect thesecond plate of the reference capacitor to the one of (a) the resetvoltage and (b) the first plate of the capacitor. Connecting the secondplate of the reference capacitor to the one of (a) the reset voltage.

Each pixel may comprise a gate line VCI, and a conduction path of thegate line VCI may connect the reference signal supply to the first plateof the reference capacitor for providing the control voltage.

An aspect also provides a method of operating a sensor array comprisinga plurality of touch sensitive pixels, the method comprising: applying acontrol voltage to a reference capacitor of a pixel of the sensor tocharge the reference capacitor and a capacitive sensing electrode,wherein the reference capacitor and the capacitive sensing electrodetogether provide, in response to the control voltage, an indicatorvoltage indicative of the proximity of a conductive object to be sensedby the pixel.

The indicator voltage may be used to control the impedance of aconduction path of a sense VCI connected to an input channel of areadout circuit. The control voltage may be provided by an outputchannel of a gate drive circuit, and the output channel of the gatedrive circuit may be connected to the input channel of the readoutcircuit via the conduction path of the sense VCI. The method maycomprise operating the sense VCI to modulate the impedance of aconduction path between the output channel of the gate drive circuit andthe input channel of the readout circuit. This may help to addressproblems associated with parasitic capacitance from other pixels in thesame column from swamping signal from the active pixel. An alternativeway to address this same technical problem is provided when a referencesignal supply is connected to the input channel of the readout circuitby the conduction path of the sense VCI. In these embodiments, themethod may comprise operating the sense VCI to modulate the impedance ofa conduction path between the reference signal supply and the inputchannel of the readout circuit.

The method may comprise operating a gate line VCI to provide the controlvoltage by connecting the reference signal supply to the referencecapacitor via a conduction path of the gate line VCI.

The method may comprise resetting the reference capacitor prior to asubsequent application of the control voltage. Resetting the referencecapacitor may comprise operating a reset circuit using a control voltagewhich is also applied to another pixel of the sensor array—for example acontrol voltage which is used to activate another row of pixels of thearray.

Resetting the reference capacitor may comprise operating a reset VCI toconnect a first plate of the reference capacitor to one of: (a) a resetvoltage; and (b) a second plate of the reference capacitor. Resettingthe reference capacitor may comprise connecting the first and secondplates of the reference capacitor to each other.

An aspect also provides an individual pixel comprising: a capacitivesensing electrode for accumulating a charge in response to proximity ofa conductive object to be sensed; a reference capacitor connected inseries with the capacitive sensing electrode so that, in response to acontrol voltage, an indicator voltage is provided at the connection (18)between the reference capacitor and the capacitive sensing electrode toindicate the proximity of the conductive object to be sensed.Embodiments may provide a group of such pixels, such as may provide acomponent part of such an array.

For the avoidance of doubt, the disclosure of this application isintended to be considered as a whole. Any feature of any one of theexamples disclosed herein may be combined with any selected features ofany of the other examples described herein.

For example, features of methods may be implemented in suitablyconfigured hardware, and the functionality of the specific hardwaredescribed herein may be employed in methods which may implement thatsame functionality using other hardware.

BRIEF DESCRIPTION OF DRAWINGS

Some practical implementations will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 comprises a plan view of a sensor apparatus comprising a sensorarray, and Inset A of FIG. 1 shows a circuit diagram for a pixel of thesensor array;

FIG. 2 shows a circuit diagram of a sensor array for a sensor apparatussuch as that illustrated in FIG. 1;

FIG. 3 shows a circuit diagram of another sensor array of the type shownin FIG. 1;

FIG. 4 shows a signal timing diagram for the operation of a pixel in asensor array of the type shown in FIG. 3;

FIG. 5 shows a circuit diagram of a pixel for use in a sensor array suchas that illustrated in FIG. 3;

FIG. 6 shows a circuit diagram of another pixel for use in a sensorarray such as that illustrated in FIG. 3.

In the drawings like reference numerals are used to indicate likeelements.

SPECIFIC DESCRIPTION

FIG. 1 shows a sensor apparatus 1 in which the sensor array 10 of thepresent disclosure may be incorporated. FIG. 2 illustrates a circuitdiagram of one such sensor array 10. The description which follows shallrefer to FIG. 1 and FIG. 2 together. It can be seen from an inspectionof FIG. 1 and FIG. 2 that inset A of FIG. 1 shows a detailed view of onepixel of this array 10.

The sensor array 10 comprises a plurality of touch sensitive pixels 12.Typically, other than in respect of its position in the array, eachpixel 12 is identical to the others in the array 10. As illustrated,each pixel 12 comprises a capacitive sensing electrode 14 foraccumulating a charge in response to proximity of the surface of aconductive object to be sensed. A reference capacitor 16 is connectedbetween the capacitive sensing electrode 14 and a connection to a gatedrive channel 24-1 of a gate drive circuit 24. Thus, a first plate ofthe reference capacitor 16 is connected to the gate drive channel 24-1,and a second plate of the reference capacitor 16 is connected to thecapacitive sensing electrode 14.

Each pixel 12 may also comprise a sense VCI (voltage controlledimpedance) 20 having a conduction path, and a control terminal (22;inset A, FIG. 1) for controlling the impedance of the conduction path.The conduction path of the sense VCI 20 may connect the gate drivechannel 24-1 to an output of the pixel 12. The control terminal 22 ofthe VCI is connected to the capacitive sensing electrode 14 and to thesecond plate of the reference capacitor 16. Thus, in response to acontrol voltage applied by the gate drive channel 24-1, the referencecapacitor 16 and the capacitive sensing electrode 14 act as a capacitivepotential divider.

The capacitance of the capacitive sensing electrode 14 depends on theproximity, to the capacitive sensing electrode 14, of a conductivesurface of an object to be sensed. Thus, when a control voltage isapplied to the first plate of the reference capacitor 16, the relativedivision of that voltage between that sensing electrode 14 and thereference capacitor 16 provides an indication of the proximity of thesurface of that conductive object to the capacitive sensing electrode14. This division of the control voltage provides an indicator voltageat the connection 18 between the reference capacitor 16 and thecapacitive sensing electrode 14. This indicator voltage can be appliedto the control terminal 22 of the sense VCI 20 to provide an output fromthe pixel 12 which indicates proximity of the conductive object.

Pixels may be positioned sufficiently close together so as to be able toresolve contours of the skin such as those associated with epidermalridges, for example those present in a fingerprint, palmprint or otheridentifying surface of the body. It will be appreciated in the contextof the present disclosure that contours of the skin may comprise ridges,and valleys between those ridges. During touch sensing, the ridges maybe relatively closer to a sensing electrode than the “valleys” betweenthose ridges. Accordingly, the capacitance of a sensing electrodeadjacent a ridge will be higher than that of a sensing electrode whichis adjacent a valley. The description which follows explains how systemscan be provided in which sensors of sufficiently high resolution toperform fingerprint and other biometric touch sensing may be providedover larger areas than has previously been possible.

As shown in FIG. 1 and FIG. 2 in addition to the sensor array 10, such asensor may also comprise a dielectric shield 8, a gate drive circuit 24,and a read out circuit 26. A connector 25 for connection to a hostdevice may also be included. This may be provided by a multi-channelconnector having a plurality of conductive lines. This may be flexible,and may comprise a connector such as a flexi, or flexi-rigid PCB, aribbon cable or similar. The connector 25 may carry a host interface 27,such as a plug or socket, for connecting the conductive lines in theconnector to signal channels of a host device in which the sensorapparatus 1 is to be included.

The host interface 27 is connected by the connector 25 to the read-outcircuit 26. A controller (6; FIG. 2) may be connected to the gate drivecircuit 24 for operating the sensor array, and to the read-out circuit26 for obtaining signals indicative of the self-capacitance of pixels ofthe sensor array 10.

The dielectric shield 8 is generally in the form of a sheet of aninsulating material which may be transparent and flexible such as apolymer or glass. The dielectric shield 8 may be flexible, and may becurved. An ‘active area’ of this shield overlies the sensor array 10. Insome embodiments, the VCIs and other pixel components are carried on aseparate substrate, and the shield 8 overlies these components on theirsubstrate. In other embodiments the shield 8 provides the substrate forthese components.

The sensor array 10 may take any one of the variety of forms discussedherein. Different pixel designs may be used, typically however thepixels 12 comprise at least a capacitive sensing electrode 14, areference capacitor 16, and at least a sense VCI 20.

The array illustrated in FIG. 2 comprises a plurality of rows of pixelssuch as those illustrated in FIG. 1. Also shown in FIG. 2 is the gatedrive circuit 24, the read out circuit 26, and a controller 6. Thecontroller 6 is configured to provide a clock signal, e.g. a periodictrigger, to the gate drive circuit 24, and to the read-out circuit 26.

The gate drive circuit 24 comprises a plurality of gate drive channels24-1, 24-2, 24-3, which it is operable to control separately, e.g.independently. Each such gate drive channel 24-1, 24-2, 24-3 comprises avoltage source arranged to provide a control voltage output. And eachchannel 24-1 is connected to a corresponding row of pixels 12 of thesensor array 10. In the arrangement shown in FIG. 2 each gate drivechannel 24-1, 24-2, 24-3 is connected to the first plate of thereference capacitor 16 in each pixel 12 of its row of the sensor array10. During each clock cycle, the gate drive circuit 24 is configured toactivate one of the gate drive channels 24-1, 24-2, 24-3 by applying agate drive pulse to those pixels. Thus, over a series of cycles thechannels (and hence the rows) are activated in sequence, and move fromone step in this sequence to the next in response to the clock cyclefrom the controller 6.

The read-out circuit 26 comprises a plurality of input channels 26-1,26-2, 26-3. Each input channel 26-1, 26-2, 26-3 is connected to acorresponding column of pixels 12 in the sensor array 10. To providethese connections, the conduction path of the sense VCI 20 in each pixel12 is connected to the input channel 26-1 for the column.

Each input channel 26-1, 26-2, 26-3 of the read out circuit 26 maycomprise an analogue front end (AFE) and an analogue-to-digitalconverter (ADC) for obtaining a digital signal from the column connectedto that input channel 26-1. For example it may integrate the currentapplied to the input channel during the gate pulse to provide a measureof the current passed through the sense VCI 20 of the active pixel 12 inthat column. The read out circuit 26 may convert this signal to digitaldata using the ADC. Furthermore, the analogue front end performsimpedance matching, signal filtering and other signal conditioning andmay also provide a virtual reference.

In the sensor array 10 shown in FIG. 2, the conduction channel of thesense VCI 20 in each pixel connects the input channel of the read outcircuit for that column to the gate drive channel for the pixel's row.In FIG. 2, the gate drive channel for the row thus provides a referenceinput. Operation of the sense VCI 20 modulates this reference input toprovide the pixel output. This output signal from a pixel indicates thecharge stored on the capacitive sensing electrode 14 in response to thatreference input relative to that stored on the reference capacitor.

FIG. 1 includes a grid as a very schematic illustration of the rows andcolumns of pixels 12 which make up the array. Typically this will be arectilinear grid, and typically the rows and columns will be evenlyspaced. For example the pixels may be square. It will of course beappreciated that the grid shown in FIG. 1 is not to scale. Typically thesensor array has a pixel spacing of at least 200 dots per inch, dpi (78dots per cm). The pixel spacing may be at least 300 dpi (118 dots percm), for example at least 500 dpi (196 dots per cm).

Operation of the sensor array 10 of FIG. 2 will now be described.

On each cycle of operation, the gate drive circuit 24 and the read outcircuit 26 each receive a clock signal from the controller 6.

In response to this clock signal, the gate drive circuit operates one ofthe gate drive channels to apply a control voltage to one of the rows ofthe array. In each pixel in the row, the control voltage from the gatedrive channel is applied to the series connection of the referencecapacitor 16 and the capacitive sensing electrode 14. The voltage at theconnection 18 between the two provides an indicator voltage indicatingthe proximity of a conductive surface of an object to be sensed to thecapacitive sensing electrode 14. This indicator voltage may be appliedto the control terminal of the sense VCI 20 to control the impedance ofthe conduction path of the sense VCI 20. A current is thus providedthrough the conduction path of the sense VCI 20 from the gate drive tothe input channel for the pixel's column. This current is determined bythe gate drive voltage, and by the impedance of the conduction channel.

In response to the same clock signal, the read-out circuit 26 senses thepixel output signal at each input channel. This may be done byintegrating the current received at each input of the read-out circuit26 over the time period of the gate pulse. The signal at each inputchannel, such as a voltage obtained by integrating the current from thecorresponding column of the array, may be digitised (e.g. using an ADC).Thus, for each gate pulse, the read-out circuit 26 obtains a set ofdigital signals, each signal corresponding to a column of the active rowduring that gate pulse. So the set of signals together represent theactive row as a whole, and the output from each pixel being indicativeof the charge stored on and/or the self-capacitance of the capacitivesensing electrode 14 in that pixel.

Following this same process, each of the gate-drive channels isactivated in sequence. This drives the sense VCI 20 of each pixelconnected to that channel into a conducting state for a selected time(typically the duration of one gate pulse). By activating the rows ofthe array in sequence the read out circuit, can scan the sensor arrayrow-wise. Other pixel designs, other scan sequences, and other types ofsensor array, may be used.

FIG. 3 illustrates another sensor array which may be used in theapparatus illustrated in FIG. 1.

FIG. 3 shows a sensor array 10 comprising a plurality of pixels, and areference signal supply 28 for supplying a reference signal to thepixels. This can avoid the need for the gate drive power supply also toprovide the current necessary for the read-out signal.

Also shown in FIG. 3 is the gate drive circuit 24, the read-out circuit26, and the controller 6.

The sensor array 10 may also benefit from the inclusion of a resetcircuit 32, 34 in each pixel. This may allow the control terminal 22 ofthe pixel to be pre-charged to a selected reset voltage whilst the pixelis inactive (e.g. while another row of the array is being activated bythe application of a gate pulse to another, different, row of thearray).

In these embodiments the sensor may also comprise a reset voltageprovider 42 for providing a reset voltage to each of the pixels 12 ofthe array as described below. The reset voltage provider 42 may comprisevoltage source circuitry, which may be configured to provide acontrollable voltage, and may be connected to the controller 6 to enablethe controller 6 to adjust and fix the reset voltage.

The reset voltage may be selected to tune the sensitivity of the pixel.In particular, the output current of the sense VCI 20 typically has acharacteristic dependence on the indicator voltage at the controlterminal 22 and its switch-on voltage. Thus the reset voltage may bechosen based on the switch-on voltage of the sense VCI 20. Thecharacteristic may also comprise a linear region in which it may bepreferable to operate.

The pixels illustrated in FIG. 3 are similar to those illustrated inFIG. 1 and FIG. 2 in that each comprise a capacitive sensing electrode14, and a reference capacitor 16 connected with a capacitive sensingelectrode 14. The connection between these two capacitances provides anindicator voltage, which can for example be connected to the controlterminal 22 of a sense VCI 20. In addition, the pixels of the sensorarray illustrated in FIG. 3 also comprise a further two VCIs 34, 38, anda connection to the reset voltage provider 42, and a connection to thereference signal supply 28.

As noted above, the sense VCI 20 is arranged substantially as describedabove with reference to FIG. 1, in that its control terminal 22 isconnected to the connection between the reference capacitor 16 and thecapacitive sensing electrode 14. However, the conduction path of thesense VCI 20 is connected differently in FIG. 3 than in FIG. 1. Inparticular, the conduction channel of the select VCI 38 connects theconduction channel of the sense VCI 20 to the reference signal supply 28which provides a voltage V_(ref). Thus, the conduction channel of thesense VCI 20 is connected in series between the conduction channel ofthe select VCI 38 and the input of the read-out circuit for the column.The select VCI 38 therefore acts as a switch that, when open, connectsthe sense VCI 20 between, V_(ref), the reference signal supply 28 andthe input of the read-out circuit and, when closed, disconnects thesense VCI from the reference signal supply 28. In the interests ofclarity, the connection between the conduction channel of the select VCIand V_(ref), the output of the reference signal supply 28 is shown onlyin the top row of the array of pixels. The connection reference signalsupply 28 in the lower rows of the array is indicated in the drawingusing the label V_(ref).

The select VCI 38 is therefore operable to inhibit the provision ofsignal from any inactive pixel to the input of the read-out circuit 26.This can help to ensure that signal is only received from active pixels(e.g. those in the row to which the gate drive pulse is being applied).

In an embodiment each column of pixels is virtually connected to aground or reference voltage. As such there may be no voltage differenceson each of the columns thereby minimising parasitic capacitance.Furthermore, the reference signal supply may apply a current-driverather than a voltage-drive which further reduces any effect parasiticcapacitance could have on the signal applied by the active pixels on theinputs of the read-out circuit 26.

The gate drive channel for the pixel row is connected to the first plateof the reference capacitor 16, and to the control terminal of a selectVCI 38. As in the pixel illustrated in FIG. 1, and FIG. 2, theconnection to the reference capacitor 16 and capacitor sensing electrode14 means that the gate drive voltage is divided between the referencecapacitor 16 and the capacitive sensing electrode 14 to provide theindicator voltage which controls the sense VCI 20. The connection to thecontrol terminal 40 of the select VCI 38 however means that, when thepixel is not active, the conduction path of the sense VCI 20 isdisconnected from the reference signal supply 28.

A control terminal 22 of the sense VCI 20 is connected to the secondplate of the reference capacitor 16. The conduction path of the senseVCI 20 connects the reference signal supply 28 to the input of theread-out circuit 26 for the pixel's column.

A conduction path of the reset VCI 34 is connected between the secondplate of the reference capacitor 16 and a voltage output of the resetvoltage provider, for receiving the reset voltage. The control terminal32 of the reset VCI 34 is connected to a reset signal provider, such asthe gate drive channel of another row of the sensor array. This canenable the reset VCI 34 to discharge the reference capacitor 16 duringactivation of another row of the array (e.g. a row of the array which isactivated on the gate pulse prior to the pixel's row) or to pre-chargethe control terminal 22 of the sense VCI 20 to the reset voltage.

Operation of the sensor array of FIG. 3 will now be described.

The gate drive circuit 24 and the read-out circuit 26 each receive aclock signal from the controller 6. In response to this clock signal,the gate drive circuit 24 activates a first gate drive channel of thegate drive circuit 24 to provide a gate pulse to a row of the array 10.A control voltage is thus applied to the control terminal of the selectVCI 38 of the pixels in the first row (the active row during this gatepulse).

The control voltage is also applied to the control terminal of the resetVCI 34 of the pixels in a second row (inactive during this gate pulse).

In the first row (the active row), the conduction channel of the selectVCI 38 is switched into a conducting state by the control voltage (e.g.that which is provided by the gate pulse). The conduction channel of theselect VCI 38 thus connects the conduction channel of the sense VCI 20to the reference signal supply 28.

The control voltage is also applied to the first plate of the referencecapacitor 16. The relative division of voltage between the sensingelectrode 14 and the reference capacitor 16 provides an indicatorvoltage at the connection between the reference capacitor 16 and thecapacitive sensing electrode 14 as described above with reference toFIG. 1 and FIG. 2. The indicator voltage is applied to the controlterminal 22 of the sense VCI 20 to control the impedance of theconduction channel of the sense VCI 20. Thus, the sense VCI 20 connectsthe reference signal supply 28 to the input channel of the read-outcircuit 26 for that column, and presents an impedance between the twowhich indicates the capacitance of the capacitive sensing electrode 14.Please note, the reference signal supply may be provided by a constantvoltage current supply.

A current is thus provided through the conduction path of the sense VCI20 from the reference signal supply 28 to the input channel of theread-out circuit 26 for the pixel's column. This current is determinedby the voltage of the reference signal supply and by the impedance ofthe conduction channel of the sense VCI.

In response to the same clock signal from the controller 6, the read-outcircuit 26 senses the pixel output signal at each input channel (e.g. byintegrating the current provided to each input channel), and digitisesthis signal. The integration time of the read-out circuit 26 may matchthe duration of the gate pulse.

Thus, in each clock cycle, the read-out 26 circuit obtains a set ofdigital signals, each signal corresponding to the signals sensed fromeach column of the active row during the gate pulse. The output fromeach pixel 12 in the row (each channel during that gate pulse) beingindicative of the charge stored on the capacitive sensing electrode inthat pixel.

In the second (inactive) row the control voltage is applied to thecontrol terminal 32 of the reset VCI 34. This causes the reset VCI 34 ofthe pixels in the inactive row to connect the second plate of theirreference capacitors 16 to a reset voltage provided by the reset voltageprovider. This may discharge (e.g. at least partially remove) chargeaccumulated on the pixels of the inactive row, or it may charge them tothe reset voltage, before they are next activated in a subsequent gatepulse. This reset voltage may be selected to tune the sensitivity of thepixels.

At the boundaries of the pixel array, where an N−1 gate line is notavailable, a dummy signal may be used to provide the control signal tothe reset VCI. The gate drive circuit 24 may provide the dummy signal.This may be provided by a gate drive channel which is only connected tothe rest VCIs of a row at the boundary of the array, but not to anysense or select VCIs.

As illustrated in FIG. 3, the reset VCI 34 of the pixels may beconnected to the gate drive circuit so that each row is discharged inthis way by the gate pulse which activates the immediately precedingrow, which may be an adjacent row of the array.

FIG. 4 shows a signal timing diagram for a pixel in row N of the sensorarray of FIG. 3 in operation.

The signal timing diagram of FIG. 4 comprises four separate plots (100,102, 104, 106) of signal values against a common time axis.

The first plot 100 is indicative of the control voltage 100 a applied togate line N−1 in response to gate pulse N−1 100 b. The second plot 102is indicative of the control voltage 102 a applied to gate line N inresponse to gate pulse N 102 b, the third plot 104 indicates the controlterminal 22 voltage of a pixel sensing a conductive object at twodifferent distances, (104 a) and (104 b) from capacitive sensingelectrode 14, and the fourth lowermost plot 106 indicates the read-outsignals (106 a and 106 b) from the pixel in of plot 104 applied to theinput of the read-out circuit 26.

The time axis of FIG. 4 has three gradations T₀, T₁ and T₂ indicatingdifferent points in time. The interval between T₀ and T₁ indicates theduration of gate pulse N−1 and the interval between T₁ and T₂ indicatesthe duration of gate pulse N. The behaviour of the plots of FIG. 4whilst the sensor array is in operation will now be discussed.

At T₀ a control voltage 100 produced in response to the gate pulse N−1100 b is applied from gate line N−1 to the control terminals of thereset VCIs 34 of the pixels in row N. This switches the reset VCIs 34 onand therefore a conduction path is opened through the reset VCIs 34between the control terminals 22 of the sense VCIs 20 and the voltageoutput of the reset voltage provider. As a result, the reset voltage isapplied to the control terminal 22 of the sense VCI 20 of the pixel ascan be seen in the plots 104 a and 104 b between T₀ and T₁ of thevoltage of the control terminal 22 of the sense VCI 20.

As mentioned in the description above the reset voltage may be selectedto tune the sensitivity of the pixels in the array. In particular, theoutput current of the sense VCIs 20 typically have a characteristicdependence on the indicator voltage at the control terminal 22 and itsswitch on voltage. Thus the reset voltage may be chosen based on theswitch on voltage of the sense VCI 20. The characteristic may alsocomprise a linear region in which it may be preferable to operate.

At T₁, after the gate pulse N−1 has finished 100 b, a gate pulse N 102 bfrom gate line N causes the gate drive circuit 24 to apply a controlvoltage 102 a to the first electrodes of the reference capacitors 16 andto the control terminals 40 of the select VCIs 38 of the pixels in rowN.

As a result of the application of the control voltage 102 to the controlterminals 40 of the select VCIs 38 a conduction path is opened betweenthe reference signal supply 28 and the sense VCIs 20 of the pixels ofrow N.

When a conductive surface is in sufficient proximity to the capacitivesensing electrode 14 of a pixel in row N, as is the case in FIG. 4, anindicator voltage 104 a, dependent on the relative division of voltagebetween the sensing electrode 14 and the reference capacitor 16, isproduced. The indicator voltage produced by the capacitive sensingelectrode and reference capacitor is applied to the control terminal 22of the sense VCI 20. A conduction path is thereby opened between thereference signal supply 28 and the read-out circuit 26.

As a result, signals such as those shown in 106 are applied to thecorresponding input of the read-out circuit 26. The magnitude of thesignal will depend on the proximity of the conductive surface.

The example shown in FIG. 4 overlays two signals (106 a and 106 b)generated by a conductive object positioned at two different distancesfrom the capacitive sensing electrode 14 of a pixel.

The two different positions of the conductive object result in twodifferent capacitances of the sensing electrode (e.g. because the dermisof the finger provides a second “plate” of a capacitor, the first“plate” of which is provided by the sensing electrode 14). The indicatorvoltages (104 a and 104 b) are applied to the control terminal 22 of thesense VCI 20 of the pixel. Therefore the impedance of the conductionpath of the sense VCI 20 is different for the two positions of theconductive surface.

The signal applied to the input of the read-out circuit 26 is dependenton the impedance of the sense VCI 20 and is therefore different for theconductive surface at two different distances from the capacitivesensing electrode 14.

This is exemplified in the control terminal voltage signals 22 (104 aand 104 b) and read out signal profiles (106 a and 106 b) shown in FIG.4.

At T₂ the gate line pulse N 102 b finishes and the gate drive circuitceases to apply a control voltage 102 a to the pixels in row N. Thecontrol voltage 102 is no longer being applied to the control terminalsof the select VCIs 38 and as a result the select VCIs of row N turn off.

The conduction paths through the select VCIs of the pixels in row Ntherefore close and a signal (106 a and 106 b) is no longer applied bythe pixel to the input of the read-out circuit.

It can be seen that some residual charge on the sensing electrode mayremain after the gate pulse for line N 102 b has finished. It will beappreciated in the context of the present disclosure that the select VCI38 may act to prevent such residual charge operating the sense VCI 20and so giving rise to spurious signal from inactive rows.

Different pixel designs may be used in arrangements such as thatdescribed with reference to FIG. 3.

FIG. 5 illustrates a circuit diagram of the sensor pixel 12 that isillustrated in the array of FIG. 3. The pixel 12 however is shown inisolation in the interests of clarity.

It can be seen that the pixel 12 comprises a reference capacitor 16, acapacitive sensing electrode 14, a sense VCI 20, a select VCI 38 and areset VCI 34.

The pixel 12 of FIG. 5 is similar to that shown in FIG. 1 and describedabove, in that a connection between the reference capacitor 16 and thecapacitive sensing electrode 14 provides an indictor voltage which maybe connected to the control terminal of the sense VCI 20.

However, the pixel 12 of FIG. 5 differs from that of FIG. 1 in that itcomprises a select VCI 38 and a reset VCI 34.

The conduction path of the select VCI 38 is connected between theconduction channel of the sense VCI 20 and a reference signal supply.Thus the sense VCI 20 is connected in series between the select VCI 38and an input of a read-out circuit 26. A gate line input of the pixel isconnected to the control terminal 40 of the select VCI 38.

The conduction path of the reset VCI 34 is connected between the secondplate of the reference capacitor 16 and a reset voltage provider. Thecontrol terminal of the reset VCI 34 is connected to a reset signalprovider input of the pixel, when the pixel is assembled into an arraythis may, for example, be a connection with a gate line of another rowof pixels in the array.

FIG. 6 illustrates yet a further example of a pixel suitable for use inan array such as that illustrated in FIG. 3. This pixel is identical tothat described in FIG. 4 other than in that it comprises a gate line VCI30, and a second reset VCI 36.

The control terminal 44 of the gate line VCI 30 is connected to the gatedrive channel for its row of the array. The conduction path of the gateline VCI 30 is connected between the reference signal supply 28 and afirst plate of the reference capacitor 16. Thus, operation of the gateline VCI 30 by the gate drive circuit 24 can connect the seriesconnection of the reference capacitor 16 and capacitive sensingelectrode 14 to the reference signal supply 28. And, when the gate lineVCI 30 is switched off the pixel disconnects from the reference signalsupply 28.

In addition, the gate line signal need not supply the current to chargethe array for readout.

As with the pixel of FIG. 5, the reset circuit of the pixel shown inFIG. 6 comprises the reset VCI 34 connected between the second plate ofthe reference capacitor 16 and a reset voltage provider 42, and alsocomprises a reset VCI 36 connected between the first plate of thereference capacitor 16 and the reset voltage provider. The controlterminal 32 of the reset VCI34 and the control terminal 46 of the resetVCI 36 are connected to a reset signal provider, such as the gate drivechannel of another row of the sensor array as described above.

It will be appreciated in the context of the present disclosure thatother reset circuits may be used. For example the conduction path of thereset VCI 34 may connect the first plate of the reference capacitor toits second plate (e.g. to short circuit the reference capacitor 16).

Operation of the pixel 12 shown in FIG. 6 in the array of FIG. 3 issimilar to that of the operation of the pixel shown in FIG. 5 anddescribed above. However in addition to operating the control terminalof the select VCI 38, the control voltage also operates the gate lineVCI 30 to connect the first plate of the reference capacitor 16 to thereference signal supply 28. This reference signal is used to provide anindicator voltage at the connection between the reference capacitor 16and the capacitive sensing electrode 14 as described above withreference to FIG. 1 and FIG. 2. The indicator voltage is applied to thecontrol terminal of the sense VCI 20, to control the impedance of theconduction channel of the sense VCI 20, which when a control voltage isapplied to select VCI 38 connects the reference signal supply 28 to theinput channel of the read out circuit for that column.

In addition, in response to a reset signal from a reset signal provider(such as the gate drive channel of another row of the sensor array asdescribed above) to the control terminals of the reset VCI 36 and resetVCI 34 a conduction path is opened between the reference capacitor 16and the reset voltage provider. This causes the reset voltage to beapplied to both electrodes of the reference capacitor 16 and thus thevoltage across the reference capacitor 16 to be zero.

The reset voltage may be selected based on the thickness of thedielectric shield and/or the dielectric constant of the shield. Forexample, the reset voltage may be chosen to ensure that the range ofcapacitance changes associated with features of an object to be sensed(e.g. the contours of human skin) provide changes in indicator voltagesfrom the capacitive potential divider 14, 16, that are sufficient tooperate the sense VCI in a measurably different way. For example, thereset voltage may be chosen based on a threshold voltage of the senseVCI and based on the thickness of the dielectric shield and/or thedielectric constant of the shield so that the ridges and valleys of thecontours of the skin both cause operation of the sense VCI in a linearregion of its current voltage characteristic.

This may be desirable as the output current of the conduction path ofthe sense VCI 20 is dependent on the indicator voltage applied to thecontrol terminal 22 and furthermore the change in the output current ofthe sense VCI 20 for a given change in control terminal voltage 22 isdependent on the value of the voltage applied to the control terminal22.

It may be advantageous to set the reset voltage so that the controlterminal 22 voltage of the sense VCI is in a voltage range where theVCI's output current varies linearly with control terminal 22 voltage.This is particularly true where the select VCI is present. It is alsopossible to select the reset voltage based on a threshold voltage of thesense VCI eg so that in a quiescent state, the sense VCI is at thethreshold. The embodiments described herein typically comprise one ormore voltage controlled impedance (VCI). Each such VCI has a controlterminal, and a conduction path the impedance of which can be controlledby a voltage applied to its control terminal. Examples of voltagecontrolled impedances include transistors such as thin film transistors(TFTs). It will also be appreciated that when implemented into thesensor as a means of achieving voltage controlled impedances thin filmtransistors would act as transconduction gates wherein the outputcurrent (I_(drain-source)) of the TFT would be dependent on the voltageacross the gate and source (V_(gate-source)) Of the TFT.

The dielectric shield may comprise a sheet or layer of insulatingmaterial such as glass or plastic. The insulating material of thedielectric shield described herein may be selected based on one or moreof the following properties: surface roughness, transparency, chemicalinertia, mechanical stiffness and robustness, dielectric constant;thermal behaviour and ease of manufacture. Suitable glass substratesinclude but are not limited to: Soda lime, borasilicate and SiO₂.Suitable polymer substrates include but are not limited to Poly Imide(PI), Polyethylene terephthalate (PET), polyethylene naphthalate (PEN).

It will be appreciated in the context of the present disclosure that thesensor apparatus may be designed to match certain timings.

The read-out of the pixel array performed by the read-out circuit may beexecuted on a frame basis, wherein each row of the pixels is read-outsequentially using a dedicated row time. Such that within a row time,all the corresponding pixels of that row are read-out.

Such a read-out is similar to TFT display technology. However thepresent case relates to pixel-reading of the sensor array rather thanthe pixel-writing of a TFT display.

The collection of all rows and related timing, may define the frame timeof the sensor apparatus. For example a row time of 25 μsec, with a totalof 100 rows, would result in a frame time of 2.5 msec i.e 25 μsec×100msecs.

Other timings and read-out sequences may also be appropriate. Forexample in the case of multiplexer circuits, a multiple number ofconversions may have to be executed within a single gate line time.

The embodiments disclosed herein describe the provision of a resetvoltage to each of the pixels in the pixel array. The pixels in thepixel array may receive the same reset voltage wherein the reset voltageis chosen in the manner described previously.

It will be appreciated from the discussion above that the embodimentsshown in the Figures are merely exemplary, and include features whichmay be generalised, removed or replaced as described herein and as setout in the claims. With reference to the drawings in general, it will beappreciated that schematic functional block diagrams are used toindicate functionality of systems and apparatus described herein. Itwill be appreciated however that the functionality need not be dividedin this way, and should not be taken to imply any particular structureof hardware other than that described and claimed below. The function ofone or more of the elements shown in the drawings may be furthersubdivided, and/or distributed throughout apparatus of the disclosure.In some embodiments the function of one or more elements shown in thedrawings may be integrated into a single functional unit.

For example, the use of a controller to provide a clock signal has beenmentioned. However, it will be appreciated in the context of the presentdisclosure that the controller may be part of the gate drive circuitand/or the gate pulse signal which is described herein may be usedinstead of the clock signal. For example, operation of the read-outcircuit and the gate drive circuit may by synchronised using the gatepulses. For example the integration period of each input channel of theread out circuit may be controlled by the gate pulse.

In some examples the functionality of the controller may be provided bya general purpose processor, which may be configured to perform a methodaccording to any one of those described herein. In some examples thecontroller may comprise digital logic, such as field programmable gatearrays, FPGA, application specific integrated circuits, ASIC, a digitalsignal processor, DSP, or by any other appropriate hardware. In someexamples, one or more memory elements can store data and/or programinstructions used to implement the operations described herein.Embodiments of the disclosure provide tangible, non-transitory storagemedia comprising program instructions operable to program a processor toperform any one or more of the methods described and/or claimed hereinand/or to provide data processing apparatus as described and/or claimedherein. The controller may comprise an analogue control circuit whichprovides at least a part of this control functionality. An embodimentprovides an analogue control circuit configured to perform any one ormore of the control methods described herein.

The above embodiments are to be understood as illustrative examples.Further embodiments are envisaged. It is to be understood that anyfeature described in relation to any one embodiment may be used alone,or in combination with other features described, and may also be used incombination with one or more features of any other of the embodiments,or any combination of any other of the embodiments. Furthermore,equivalents and modifications not described above may also be employedwithout departing from the scope of the invention, which is defined inthe accompanying claims.

1. A sensor array comprising a plurality of touch sensitive pixels, eachpixel comprising: a capacitive sensing electrode for accumulating acharge in response to proximity of a conductive object to be sensed; areference capacitor connected in series with the capacitive sensingelectrode so that, in response to a control voltage, an indicatorvoltage is provided at the connection between the reference capacitorand the capacitive sensing electrode to indicate the proximity of theconductive object to be sensed, a sense voltage controlled impedance,VCI, having a control terminal connected so that the impedance of thesense VCI is controlled by the indicator voltage wherein each pixelcomprises a reset circuit for setting the control terminal of the senseVCI to a reset voltage selected to tune the sensitivity of the pixels.2. The sensor array of claim 1 wherein a control terminal of the resetcircuit is connected to another pixel of the sensor for receiving areset signal.
 3. The sensor array of claim 1 wherein the reset circuitcomprises a reset VCI wherein a conduction path of the reset VCI isconnected between a second plate of the reference capacitor and a resetvoltage.
 4. The sensor array of claim 1 wherein the reset voltage isprovided by a reset voltage provider and is based on at least one of: aswitch on voltage of the sense VCI; a thickness and/or dielectricconstant of a dielectric touch shield of the sensor; and a linear regionof operation of the sense VCI.
 5. The sensor array of claim 1 whereinthe reset circuit comprises a reset VCI wherein a conduction path of thereset VCI is connected between a second plate of the reference capacitorand a first plate of the reference capacitor.
 6. The sensor array ofclaim 3 wherein the reset VCI comprises two conduction paths arranged toconnect both the first and second plates of the reference capacitor (16)to a reset voltage.
 7. The sensor array of claim 1 wherein a conductionpath of the sense VCI is connected to a first plate of the referencecapacitor, and the control terminal of the first VCI is connected to thesecond plate of the reference capacitor.
 8. The sensor array of claim 7wherein the conduction path of the sense VCI connects the first plate ofthe reference capacitor to an input of a readout circuit.
 9. The sensorarray of claim 1 wherein a conduction path of the sense VCI connects areference signal supply to an input of a readout circuit.
 10. The sensorarray of claim 9, comprising a select VCI having a conduction pathconnected in series between the conduction path of the sense VCI and thereference signal supply.
 11. The sensor array of claim 9, wherein acontrol terminal of the select VCI is connected to the control voltage.12. The sensor array of claim 5 wherein each pixel comprises a gate lineVCI, and a conduction path of the gate line VCI connects the referencesignal supply to the reference capacitor for providing the controlvoltage. 13-24. (canceled)
 25. The sensor array of claim 1 wherein thevoltage controlled impedances are constructed on a glass or polymersubstrate, which substrate separates the capacitive sensing electrodefrom the object to be sensed.
 26. An adjustable capacitive touch sensor,comprising a plurality of touch sensitive pixels, each comprising acapacitive potential divider arranged to provide capacitive touchsensing and a reset voltage provider arranged to pre-charge thecapacitive potential divider thereby to adjust sensitivity of the touchsensitive pixels of the sensor.
 27. The capacitive touch sensor of claim26 wherein the plurality of touch sensitive pixels each comprise a senseVCI wherein the control terminal of the sense VCI is connected to thecapacitive potential divider arranged to provide an output signaldependent on an indicator voltage provided by the capacitive potentialdivider.
 28. The capacitive touch sensor of claim 26 wherein theplurality of touch sensitive pixels each comprise a reset VCI wherein aconduction path of the reset VCI is connected in series between thecapacitive potential divider and a reset voltage provider forpre-charging the potential divider.
 29. A method of calibrating anadjustable capacitive touch sensor, comprising a plurality of touchsensitive pixels, each comprising a capacitive potential dividerarranged to provide capacitive touch sensing and a reset voltageprovider arranged to pre-charge the capacitive potential divider therebyto adjust sensitivity of the touch sensitive pixels of the sensor, themethod comprising: applying a reset voltage to the potential divider ofat least one pixel to pre-charge the potential divider.
 30. The methodof claim 29 comprising selecting the reset voltage based on at least oneof: a switch on voltage of the sense VCI; a thickness and/or dielectricconstant of a dielectric touch shield of the sensor; and a linear regionof operation of the sense VCI.
 31. The method of claim 29 wherein the atleast one pixel comprises: a capacitive sensing electrode foraccumulating a charge in response to proximity of a conductive object tobe sensed; a reference capacitor connected in series with the capacitivesensing electrode so that, in response to a control voltage, anindicator voltage is provided at the connection between the referencecapacitor and the capacitive sensing electrode to indicate the proximityof the conductive object to be sensed.
 32. The method of claim 31,wherein the at least one pixel comprises a reset circuit for setting theconnection between the capacitive sensing electrode and the referencecapacitor to the selected reset voltage.
 33. (canceled)
 34. (canceled)